1. Field of the Invention
The present invention relates to a system for treating waste water by nitrification and, in particular, to a waste water nitrification treating system using a carrier which is formed by fixing nitrifying bacteria.
2. Description of the Related Art
As a biological treating method for treating organic waste water (which is hereinafter simply referred to as waste water) including nitrogen, there is known an activated sludge circulation method. According to this method, there are arranged a denitrification vessel, a nitrifying vessel and a sedimentation vessel, respectively. Specifically, waste water supplied to the denitrification vessel is affected by denitrifying bacteria under anaerobic conditions in the denitrification vessel so that the nitrogen component of the waste water is caused to turn into nitrogen by means of a reaction NO.sub.3.sup.- .fwdarw.N.sub.2 to be removed. On the other hand, a treating solution in the denitrification vessel is affected by nitrifying bacteria under an aerobic condition in the nitrifying vessel so that the nitrogen component in the treating solution is caused to change from NH.sub.3.sup.+ .fwdarw.NO.sub.3.sup.-. The treating solution in the nitrifying vessel, for the most part, is returned through a nitrifying solution circulation line to the denitrification vessel and the nitrogen component NO.sub.3.sup.- of the treating solution of the nitrifying vessel is again caused to turn into nitrogen by means of the above-mentioned reaction in the denitrification vessel and is then removed from the solution. Part of the treating solution of the nitrifying vessel is supplied to the sedimentation vessel. The thus obtained solution is discharged and the sedimented sludge, for the most part, is returned to the above-mentioned denitrification vessel while the remaining portion thereof is disposed as an excess sludge. And, in this treating process, the BOD component of the waste water is also dissolved and removed. The above-mentioned treating method, similarly to a so-called an activated sludge treating method, is a system in which an activated sludge (an aggregate of microorganisms) contributing to reaction floats and circulates, and in this system the nitrifying bacteria that are slow in multiplication rate are easy to flow out from the system as an excess sludge and the activity of the bacteria is greatly lowered in colder water temperatures. In order to solve this problem, there has recently been made a study to enhance the reaction speed by fixing nitrifying bacteria and holding the fixed bacteria in a high density. As a result of this, there is proposed a treating method (Japanese Patent Application Laid-open (TOKKAI) No. 58-40198, West German Patent Application No. P3131989.0) in which a carrier formed by fixing nitrifying bacteria in high molecular water-containing gel is put into a nitrifying vessel according to the activated sludge circulation method so as to be able to remove nitrogen efficiently.
However, in the conventional nitrifying bacteria fixing methods, since the circulation of a nitrifying solution from a nitrifying vessel to a denitrification vessel is made by use of a pump, a power required for treatment is great, which provides another problem to be solved.